The difference is that Microsoft very intentionally decided it was going to make .NET cross-platform as a strategic play. Apple has recently shown a willingness to get out of the way of cross-platform Swift, but I haven't yet seen evidence that they're throwing any significant weight behind it. They have an even worse track record than Microsoft did of investing heavily in locking developers into their ecosystem, so I'll have to see quite a bit of evidence to believe they're actually serious about cross-platform Swift.